1. Field of the Invention
This invention relates to semiconductor devices in which charge is manipulated, and in particular, to a charge-coupled device ("CCD") in which a charge collection circuit collects all of an input signal at low signal levels, but collects less of the input signal at higher signal levels, thereby extending its dynamic range.
2. Description of the Prior Art
Charge-coupled devices, bucket brigade devices and other semiconductor devices in which charge is manipulated are now well known. Such devices are used as shift registers, as memory cells, in analog signal processing as filters, and as charge-coupled imaging devices, both for line scan and area imaging purposes. The operation of these devices is similar in that depletion regions are created in a semiconductor substrate by application of an electric field. Packets of charge are then manipulated in the depletion regions. In charge-coupled devices an initial charge is injected electrically (memory cells) or results from electron-hole pair generation (imaging devices) as a result of photon bombardment. Charge-coupled imaging devices allow optical integration of ambient electromagnetic radiation over a period of time. The resulting image/charge information then is transferred out of the regions where it accumulated into shift registers, or other means, so that the accumulated charge may be detected and amplified, or used to provide other signals.
In a typical prior art charge-coupled device, charge accumulates in a semiconductor substrate in a potential well (depletion region), defined by an overlying photogate electrode. Should the three-dimensional well beneath the photogate become saturated with charge, the charge carriers spread away from the light-sensing element and "blooming" occurs. Blooming is the spreading of the charge originally accumulated in a light-sensing element in such a way that this charge appears to have originated in other nearby light-sensing elements. This, adjacent to the photogate electrode is an antiblooming structure to absorb charge greater than the maximum amount which may accumulate under the electrode to prevent that excess charge from overflowing into surrounding photogates and destroying the image. A transfer gate electrode and transfer structure ae formed adjacent an opposite edge of the photogate. During the optical integrated period, the transfer gate is maintained at a potential to confine the accumulating charge beneath the photogate. When the optical integrated time ends, a signal is applied to the transfer gate to lower the electrical "barrier" to the transfer gate induces in the substrate, and allow the accumulated charge to flow beneath the transfer gate into an adjoining shift register element, or the like. The transfer gate then is returned to its original position to allow further accumulation of charge beneath the photogate. Meanwhile, a shift register is to transfer the charge packet to a sense amplifier or other apparatus for measurement or subsequent processing.
An unfortunate limitation on prior art charge-coupled devices is the relationship between the amount of charge which may be accumulated beneath the photogate and the charge-handling capability required of all of the peripheral circuitry, such as the shift register. Increasing the size of the photogate allows more charge to accumulate over a given period; however, such an increase in size also places a correspondingly higher requirement on the charge-handling capability of the shift register or other circuitry. Thus, the design of charge-coupled devices of a size sufficient to provide wide dynamic range is undesirably increased by the extra charge-handling capability required. The increase in size of the shift registers and associated peripheral circuitry undesirably increases the size of the die involved, thereby lowering the yield of functional devices and increasing the price for each device. Viewed alternatively, the increased size of the peripheral charge-handling circuitry, decreases the amount of chip area available for photosensing, and thereby lowers the dynamic range of the device.